Process of recovering aromatic hydrocarbons by solvent extraction



Dec. 27, 1966 R. L. DONOVAN PROCESS OF RECOVERING AHOMATIC HYDROCARBONS BY SOLVENT EXTRACTION Filed Nov. 29, 1965 $530... 2.3 m I 252 235m m I mm N 1 MN, m 4 3 =o:uoE 5:300 om 5202,5 @N Q /m QD QxUE wzom I N 0 C m 55 7 Q INVENTOR. ROY L. DONOVAN MW TORNEY United States Patent 3,294,679 PROCESS OF RECOVERENG ARQMATIC HYDRO- CARBONS BY SOLVENT EXTRACTKON Roy L. Donovan, North Hills, Wilmington, DeL, assignor to Sun Gil Company, Philadelphia, Pa, a corporation of New Jersey Filed Nov. 29, 1965, Ser. No. 510,286 3 Claims. ((Il. 208-333) This application is a continuation-in-part of application Serial No. 64,594, filed October 24, 1960 now abandoned.

This invention relates to a process of recovering aromatic hydrocarbons by solvent extraction. It particularly relates to improvements in a combination solvent extraction-rich solvent stripping process whereby increased yields of aromatic hydrocarbons are obtained. It especially relates to a process for recovering benzene, toluene, and an aromatic fraction suitable for gasoline blending by solvent extraction using diethylene glycol as the solvent.

Hydrocarbons occur naturally as mixtures of various classes, namely, aromatic, olefinic, paraffinic and naphthenic. Other compounds containing sulfur and/or oxygen can also be present. It is known that these mixtures may be separated into relatively pure classes using selective solvents flowing countercurrently to the mixed feed. For example, the aromatic hydrocarbons may be separated using the polyalkylene glycol solvents, e.g., diethylene glycol. The glycol solvents are substantially immiscible with the predominantly non-aromatic raffinate. Usually, the solvent is separated from the extract by distillation which can be performed under vacuum, steam, atmospheric, or superatmospheric pressure. These processes, extraction and distillation, normally does not give separations. The various glycol solvents which are used to extract aromatics from other hydrocarbons differ in their selectively and absorbing power for the aromatics. Therefore, depending upon the extraction conditions, the extract may contain substantially all or only a part of the aromatics which are present in the feed. Further, these solvents also have some absorbing capacity for non-aromatics which renders the recovery of only pure aromatics from the extraction step alone virtually impractical.

For these reasons, the subsequent rich solvent stripping process conventionally takes an overhead fraction comprising essentially the non-aromatics, an intermediate fraction comprising essentially aromatics, and a residue fraction comprising essentially solvent. However, it is well known that these distillation steps are relatively ineflicient and, therefore, appreciable amounts of the aromatic hydrocarbons are usually found in the overhead fraction. Further, these inefficiencies in the distillation process are tolerated because such inefficiencies can only be corrected by superfracti-onators which are very expensive.

It is also known that the extraction efliciency can be improved by injecting a substantially paraflinic hydrocarbon into the extraction zone in contact with the extract in order to displace non-aromatics from the extract. Usually, this parafiinic stream is a relatively pure C to C hydrocarbon.

The object of the present invention is to provide a process for the economical and substantially complete recovery of aromatic hydrocarbons from hydrocarbon mixtures. Another object is to provide improvements in the combination solvent extraction-rich solvent stripping process whereby increased yields of aromatic hydrocarbons are obtained. A further object is to provide improvements in the process for recovering benzene and toluene using diethylene glycol as the solvent. A specific 3,294,679 Patented Dec. 27, 1966 object is to provide a process whereby the hydrocarbon employed as reflux in the extraction zone is recovered from the extract.

The present invention is based on the discovery that the overhead fraction from the rich solvent stripping process can be subjected to distillation for recovery of a relatively rich aromatic component, usually aromatic, and of a relatively lean aromatic content component which is of increased quality for use as reflux in the extraction zone. Thus, a specific embodiment of this invention features the recovery of benzene, toluene, and an aromatic fraction suitable for gasoline blending. This aromatic fraction was found to have a research octane number in excess of without the addition of tetraethyl lead.

The present invention in its preferred embodiment is essentially an improvement of the process described and claimed in US. Patent No. 2,730,558 which should be referred to as prior art background for the present invention.

It will be obvious, however, and it is intended, that the invention described and claimed herein is applicable to other similar extraction processes.

Briefly, the process comprises introducing the mixed feed into a liquid phase multistage countercurrent extraction zone. The diethylene glycol solvent is introduced into one end of the zone and a raflinate phase comprising essentially non-aromatic components is withdrawn from the same end. The extract phase comprising mainly the solvent and the aromatic components is withdrawn from the other end and subjected to a first distillation step in which three fractions are taken. The residue from the distillation step comprises essentially diethylene glycol which is then returned to the extraction zone. The overhead fraction comprises non-aromatics with an appreciable amount of aromatics present as contaminants. Conventionally, this mixed overhead stream is returned as reflux to the extract end of the extraction zone. In this invention, the overhead fraction is subjected to a second distillation step wherein the residue fraction is relatively rich in aromatic, i.e. up to 100% aromatics, and the over-head is relatively lean in aromatics. This relatively lean aromatic fraction is returned to the extract end of the extractor as reflux. The relatively rich aromatic residue is sent to gasoline blending. The primary aromatic product from the first distillation step is withdrawn as an intermediate fraction and is sent to other processing units for separation, purification, and recovery of the desired aromatic hydrocarbons.

The preferred solvent for this invention is diethylene glycol. As herein defined, however, the term diethylene glyco means alone or in aqueous form with up to 10 percent by weight of water. Other polyalkylene glycol solvents may be used. For example, ethylene glycol, triethyleneglycol, tetraethyleneglycol, di-propyleneglycol and mixtures of these with each other or with diethyleneglycol and the like are satisfactory solvents.

The invention may be best understood by reference to the accompanying drawings which is a flow diagram of the preferred embodiment of the invention.

Referring to the drawing, liquid hydrocarbons containing benzene and toluene, i.e., comprising a mixture of aromatic and non-aromatic components, is introduced under a pressure in excess of 200 psi. through line 10 into a countercurrent multistage extractor 11 at a point near the middle of the extractor. Aqueous diethyleneglycol containing for example, from 5 to 10 percent by weight of water is introduced through line 17 into extractor 11 at a point near one end of the extractor. The solvent is introduced at a temperature between about 100 F. and 500 F., preferably between 300 F. and 350 F. The

solvent, being of greater specific gravity than the hydrocarbon feed stock, fiows down the extractor in countercurrent fashion to the rising hydrocarbon stream. A raffinate phase comprising essentially non-aromatic components is withdrawn by means of line 12. The extract phase comprising mainly aromatic components (benzene and toluene) and diethylene glycol is withdrawn through line 13. This stream is usually referred to as the rich solvent. This extract phase also contains a minor quantity of non-aromatic hydrocarbons. In addition, a reflux stream comprising the hereinafter specified relatively lean aromatic fraction having a lower boiling point than the solvent, is introduced via line 26 in contact with the extract phase in the lower end of the extraction column 11. In other words, the rich solvent is contacted with the displacing agent just prior to the removal of the rich solvent phase from the extraction column. This volatile hydrocarbon reflux is used as a displacing agent for the non-aromatics in the extract and comprises 81% aromatics. It should be noted at this point that the relatively lean aromatic displacing agent or reflux stream is the sole reflux used on the column. There is no light parafiin hydrocarbon reflux as described in US. Patent No. 2,730,558, column 6.

The extract phase or rich solvent stream comprising aqueous diethyleneglycol having dissolved therein the aromatic components, principally, benzene and toluene, and a small amount of non-aromatic hydrocarbons is sent via line 13 to the upper portion of the solvent stripping column 16 which is designated as zone A. The extract phase is charged into zone A at a temperature above the boiling points of the solute constituents dissolved in the solvent and under super-atmospheric pressure, thereby obtaining flash vaporization of the dissolved constituents since zone A is maintained at a lower pressure than the extract stream. The flashed vapors comprising the residual nonaromatic components, a portion of the aromatic stream, and water from the solvent are passed from zone A via line 19.

The rich solvent residue is transferred via line 15 into the lower portion of strippingcolumn 16 designated herein as zone B. The pressure in zone B is maintained at a lower level than that in zone A. Consequently, additional flash vaporization of volatile constituents occurs and the resulting vapors are removed from zone B via line 20.

Additional light aromatics and Water are removed through lin e22 through line 22 and are combined with the vapors from lines 19 and 20. These combined vapor streams are condensed (condenser not shown) and the hydrocarbon and water layers separated (accumulator not shown). The hydrocarbon layer has the following typical composition:

Volume Percent Total Aromatics: 86.2

Benzene 39. 42. 8 2. 3 0. 6 1. 0 o-Xylene Trace 86. 2 Non-Aromatics 13. 8

The above hydrocarbon layer is charged via line 21 into fractionator 23. An aromatic fraction comprising 26% of the charge and having the following properties is removed through line 25 and sent to gasoline blending:

Boiling Range 231 F.+

F-l Octane Number, Clear 110 Total Aromatics: 100 Benzene Trace Toluene 90.9

Ethyl benzene 0.9 p-Xylene 2.1 m-Xylene 3 .5 o-Xylene 2.6

The overhead fraction comprising 74% of the charge having a lower boiling point than the solvent, and having the following composition is removed via line 26 and recycled as reflux to the lower portion of extractor 11:

The benzene-toluene concentrate is removed through line 24 of the stripping column 16 and sent to separation, purification, and recovery units (not shown).

A lean diethyleneglycol solvent is removed from stripping column 16 via line 17 and is recycled to the upper portion of the solvent extraction column 11. In the event that the solvent volume and/ or its water content are not at the desired level, make-up can be added to line 17 through line 18.

The net effect of operating the combination solvent extraction-rich solvent stripping process according to the present invention is to provide for increased throughput of feed material due to the decreased recycle and reflux quantities and the recovery of high octane aromatic product suitable for gasoline blending in addition to the usual production of benzene and toluene.

In the process of the invention, the distillate fraction 26 from the second distillation contains a major amount, i.e. more than 50%, of aromatics and a minor amount of non-aromatics. Typically, in a process for production and recovery of benzene and toluene, the overhead fraction contains at least 40% benzene, e.g. 40 to 65% benzene, at least 15%, e.g. 15 to 40% toluene, and 10 to 30% non-aromatics. In such operation, the overhead 19, 20 and 22 from the first distillation contains mainly benzene, toluene and nonaromatics, e.g. 20 to 60% benzene, 25 to 65% toluene and 5 to 25% non-aromatics. In the second distillation, a substantial portion of the toluene is recovered as a residue, while the rest of the toluene, together with substantially all of the benzene and non-aromatics, is recovered as overhead and recycled to the extraction zone.

The invention claimed is:

1. Method of separating aromatic hydrocarbons from a feed stock boiling mainly in the range of -250" F. and composed of aromatic and non-aromatic hydrocarbons which comprises countercurrently contacting said feed in an extraction zone with a polyalkylene glycol solvent selective for aromatics, separately removing reffinate and extract phases from the extraction zone, distilling said extract phase in a first distillation zone to obtain fractions consisting essentially of (1) an overhead fraction composed of aromatics and a minor amount of non-aromatics, (2) an intermediate fraction comprising essentially high purity aromatics and (3) a residue comprising said solvent, returning the solvent residue to the extraction zone, distilling said overhead fraction in a second distillation zone to obtain fractions consisting essentially of (l) a distillate composed of aromatics and most of the non-aromatics in said overhead fraction, said distillate containing a major amount of aromatics but a lesser amount of aromatics than in said overhead fraction and (2) a residue product comprising essentially high purity aromatics', and recycling said distillate to the extraction zone as sole reflux at a locus intermediate the feed point and the point of removal of the extract phase from said extraction zone.

2. Method according to claim 1 wherein said polyalkylene glycol solvent is diethylene glycol.

3. Process according to claim 1 wherein said overhead fraction contains benzene, toluene and non-aromatics, and said distillate contains 40 to 65% benzene, 15 to 40% toluene and 10 to 30% non-aromatics.

References Cited by the Examiner UNITED STATES PATENTS 2,727,848 12/1955 Georgian 208314 2,794,839 6/ 1957 Broughton 260674 2,904,507 9/1959 Hughes et a1 260674 DELBERT E, GANTZ, Primary Examiner.

HERBERT LEVINE, Assistant Examiner. 

1. METHOD OF SEPARATING AROMATIC HYDROCARBONS FROM A FEED STOCK BOILING MAINLY IN THE RANGE OF 125-250* F. AND COMPOSED OF AROMATIC AND NON-AROMATIC HYDROCARBONS WHICH COMPRISES COUNTERCURRENTLY CONTACTING SAID FEED IN AN EXTRACTION ZONE WITH A POLYALKYLENE GLYCOL SOLVENT SELECTED FOR AROMATICS, SEPARATELY REMOVING RAFFINATE AND EXTRACT PHASES FROM THE EXTRACTION ZONE, DISTILLING SAID EXTRACT PHASE IN A FIRST DISTILLATION ZONE TO OBTAIN FRACTION CONSISTING ESSENTIALLY OF (1) AN OVERHEARD FRACTION COMPOSED OF AROMATICS AND A MINOR AMOUNT OF NON-AROMATICS, (2) AN INTERMEDIATE FRACTION COMPRISING ESSENTIALLY HIGH PURITY AROMATICS AND (3) A RESIDUE TO THE EXTRACTION ZONE, VENT, RETURNING THE SOLVENT RESIDUE TO THE EXTRACTION ZONE, DISTILLING SAID OVERHEAD FRACTION IN A SECOND DISTILLATION ZONE TO OBTAIN FRACTIONS CONSISTING ESSENTIALLY OF (1) A DISTILLATE COMPOSED OF AROMATICS AND MOST OF THE NON-AROMATICS IN SAID OVERHEAD FRACTION, SAID DISTILLATE CONTAINING A MAJOR AMOUNT OF AROMATICS BUT A LESSER AMOUNT OF AROMATICS THAN IN SAID OVERHEAD FRACTION AND (2) A RESIDUE PRODUCT COMPRISING ESSENTIALLY HIGH PURITY AROMATICS, AND RECYCLING SAID DISTILLATE TO THE EXTRACTION ZONE AS SOLE REFLUX AT A LOCUS INTERMEDIATE THE FEED POINT AND THE POINT OF REMOVAL OF THE EXTRACT PHASE FROM SAID EXTRACTION ZONE. 